Some transmission systems use frequency bands which are very sensitive to interference. This is especially the case for systems operating at around 5 GHz. Many solutions are known to make the transmission useable. Among the known solutions, the use of multi-carrier modulation with information redundancy makes it possible to obtain good transmission performance. It is also known to use several antennas by selecting the one which receives the best quality signal on reception.
A known receiving system uses two switched antennas which use the same receiving channel. Provision is made to receive two successive test bursts in order to evaluate the two receiving paths before receiving a transmission burst. While on standby for reception, an evaluation circuit positions a switch in order to connect a first antenna to the input of an amplifier with automatic gain control. On receiving the first test burst, the power of the signal received in the evaluation circuit is measured. The power is measured by recovering the gain reference value of the amplifier with automatic gain control. The evaluation circuit then positions the switch in order to connect a second antenna to the input of the amplifier so as to measure the power received by the second antenna. At the end of the two test bursts, the evaluation circuit positions the switch on the antenna which corresponds to the strongest signal received.
In the case of multi-carrier modulation, it is possible that the distribution of the signal over the various carriers is not uniform. The measurement of total power of the signal may therefore not indicate the real performance of the transmission. Also, the selection of the receiving path may not be limited to the power of the signal received.
Another technique consists in using an error correction circuit so as to count the number of errors corrected for each path. However, such a technique requires an extremely long sequence in order to select the path. Now, increasing the time for evaluating the paths reduces the overall transmission bit rate. Furthermore, such an evaluation is made on a corrected error rate but does not make it possible to determine whether a path may be at the limit of the correctability.